[PATCH 2/7] build: silence gcc warning

[Eric Blake]

Our next gnulib update will automatically turn on -Wformat=2, which
overrides our attempt to avoid -Wformat-nonliteral.  But for newer
gcc, we don't need to blindly ignore the warning; instead, we can
target just the audited code while leaving the warning to apply to
the rest of the code.  And for older gcc, we just don't use the
warning (that is, warnings are a moving target, and it is fine if
we only target newer gcc for full warnings).

* configure.ac (M4_cv_gcc_pragma_push_works): New test.
(-Wformat-nonliteral): Disable for older gcc.
* src/format.c (expand_format): Mark our only uses of nonliteral
format as being safe.

Signed-off-by: Eric Blake <address@hidden>
---
 ChangeLog    |  6 ++++++
 configure.ac | 21 ++++++++++++++++++++-
 src/format.c |  9 +++++++++
 3 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/ChangeLog b/ChangeLog
index a1dfb77..1d2400a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,11 @@
 2012-09-01  Eric Blake  <address@hidden>

+       build: silence gcc warning
+       * configure.ac (M4_cv_gcc_pragma_push_works): New test.
+       (-Wformat-nonliteral): Disable for older gcc.
+       * src/format.c (expand_format): Mark our only uses of nonliteral
+       format as being safe.
+
        doc: fix misuse of @xref
        * doc/m4.texinfo (Inhibiting Invocation): Reword.

diff --git a/configure.ac b/configure.ac
index 8eecef3..cedb922 100644
--- a/configure.ac
+++ b/configure.ac
@@ -57,6 +57,20 @@ if test "$gl_gcc_warnings" = yes; then
   gl_WARN_ADD([-Werror], [WERROR_CFLAGS])
   AC_SUBST([WERROR_CFLAGS])

+  # We use '#pragma GCC diagnostic push' to silence some
+  # warnings, but older gcc doesn't support this.
+  AC_CACHE_CHECK([whether pragma GCC diagnostic push works],
+    [M4_cv_gcc_pragma_push_works], [
+    save_CFLAGS=$CFLAGS
+    CFLAGS='-Wunknown-pragmas -Werror'
+    AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
+      #pragma GCC diagnostic push
+      #pragma GCC diagnostic pop
+    ]])],
+    [M4_cv_gcc_pragma_push_works=yes],
+    [M4_cv_gcc_pragma_push_works=no])
+    CFLAGS=$save_CFLAGS])
+
   nw=
   nw="$nw -Waggregate-return"       # C90 is anachronistic
   nw="$nw -Wlong-long"              # C90 is anachronistic
@@ -65,7 +79,6 @@ if test "$gl_gcc_warnings" = yes; then
   nw="$nw -Wsystem-headers"         # Don't let system headers trigger warnings
   nw="$nw -Wpadded"                 # Our structs are not packed
   nw="$nw -Wredundant-decls"        # Gnulib has multiple decls
-  nw="$nw -Wformat-nonliteral"      # Needed in builtin.c
   nw="$nw -Wunreachable-code"       # Needed in output.c
   nw="$nw -Wconversion"             # Too many warnings for now
   nw="$nw -Wsign-conversion"        # Too many warnings for now
@@ -80,6 +93,12 @@ if test "$gl_gcc_warnings" = yes; then
     gl_WARN_ADD([$w])
   done

+  # Gnulib turns on -Wformat=2, which implies -Wformat-nonliteral, but
+  # we temporarily override it in format.c if possible.
+  if test $M4_cv_gcc_pragma_push_works = no; then
+    gl_WARN_ADD([-Wno-format-nonliteral])
+  fi
+
   gl_WARN_ADD([-fdiagnostics-show-option])
   gl_WARN_ADD([-funit-at-a-time])

diff --git a/src/format.c b/src/format.c
index 4b7f022..940021a 100644
--- a/src/format.c
+++ b/src/format.c
@@ -348,6 +348,12 @@ expand_format (struct obstack *obs, int argc, token_data 
**argv)
       *p++ = c;
       *p = '\0';

+      /* Our constructed format string in fstart is safe.  */
+#if 4 < __GNUC__ + (6 <= __GNUC_MINOR__)
+# pragma GCC diagnostic push
+# pragma GCC diagnostic ignored "-Wformat-nonliteral"
+#endif
+
       switch (datatype)
         {
         case CHAR:
@@ -373,6 +379,9 @@ expand_format (struct obstack *obs, int argc, token_data 
**argv)
         default:
           abort();
         }
+#if 4 < __GNUC__ + (6 <= __GNUC_MINOR__)
+# pragma GCC diagnostic pop
+#endif

       /* NULL was returned on failure, such as invalid format string.  For
          now, just silently ignore that bad specifier.  */
-- 
1.7.11.4